The present invention generally relates to power boosters for brake systems, such as those used in automobiles, for intensifying the input force between a brake pedal and a master cylinder.
Brake power boosters generally utilize fluid pressure, or differentials thereof, to provide a power assist in applying force to the master cylinder of the brake system. Upon application of an input force on the brake pedal, an input member such as a pushrod activates the power booster. The power booster intensifies the force by a calibrated amount and transfers the force to a power piston which then moves the master cylinder to apply the brakes at each wheel. Power boosters also include a mechanism for transferring the feel of the brake operation back from the master cylinder through the power booster to the brake pedal. This allows the vehicle operator to feel whether they are applying more or less force with their foot. A typical mechanism includes a reaction body and a reaction disc each carried within the power piston. When the brakes are applied, a reaction force is transmitted back through the power piston and reaction disc. The reaction disc is resilient so that it may be compressed and partially extruded through the annular reaction body thereby transferring a portion of the total output force back to the brake pedal.
In conventional power boosters, an air valve assembly is opened upon depression of the brake pedal by the operator to admit atmospheric air to at least a first chamber of the power booster housing. This creates a pressure differential across a diaphragm separating the first chamber from a second chamber of the housing. The diaphragm is coupled to the power piston and transmits a force resulting from the pressure differential to the power piston and, ultimately, to the master cylinder. During normal brake application rates, the air valve assembly operates as described and the resilient reaction disc provides a reaction or feedback force to the operator through the air valve assembly to the brake pedal. However, during high rate or so-called panic apply rates, the lag of the power booster forces the air valve assembly into a run out condition. In this condition, the operator is essentially applying force directly to the power piston and the master cylinder until such time as the pressure differential of the power booster takes over with a higher applied force. Thus, during this run out period, the operator must apply an extremely high level of force to the brake pedal to move the power piston itself instead of simply moving the air valve assembly to an open position.
For these general reasons, it would be desirable to provide a power booster that maintains a high output force to the master cylinder with a reduced input force from the vehicle operator especially in high rate or panic applications of force to the brake pedal.
The present invention generally provides a power booster for a brake system with an air valve assembly having the ability to shorten in length during a high rate application of the brakes. This allows the power booster to maintain a high output force with a reduced input force on the brake pedal. The invention also improves pressure response of the brake system over conventional brake systems as the operator can apply higher forces with a lower input force on the brake pedal. This can lead to shorter vehicle stopping distances.
Generally, the power booster of this invention includes a housing having an interior and a movable diaphragm separating the interior of the housing into at least two chambers. A power piston is coupled for movement with the diaphragm and includes an output member. A resilient reaction disc is coupled to the power piston and an annular reaction body is also coupled to the power piston and positioned adjacent to the resilient reaction disc. An input member is adapted to be coupled to a movable brake pedal and is coupled to an air valve assembly. The input member moves the air valve assembly between open and closed positions to selectively admit atmospheric air into at least one of the chambers to induce an output force on the diaphragm which is transferred to the output member of the power piston.
In accordance with the invention, the air valve assembly is length adjustable between the input member and the resilient reaction disc. Preferably, one end of the air valve assembly selectively engages the resilient reaction disc through the annular reaction body. The air valve assembly operates at a first length when an input force is applied to the input member at a first rate to move the air valve assembly to the open position. The air valve operates at a second, shorter length when the input force is applied by the operator at a second, higher rate thereby lowering the input force needed to maintain a desired output force.
In another aspect of the invention, a button member is positioned within the annular reaction body and stops against a portion of the reaction body when the air valve is operating at the second, shorter length. This prevents transmission of a reaction force to the operator from the reaction disc.
Various objectives, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings.